LETTER OF TRANSMITTAL. 



Department of the Interior, 



TJ. "S. Geological Survey, 

 Division of Geologic Correlation, 



Washington, D. C, March 15, 1891. 



Sir : I have the honor to transmit herewith a memoir by Dr. Henry 

 S. Williams on the Devonian and Carboniferous formations of North 

 America, prepared for publication as a bulletin. 



This memoir is the first of a series, and in order to show its relation 

 to those which are to follow, I quote the following passage from the 

 report of the Director for the fiscal year ending June 30, 1888 : 1 



In order to develop the geological history of the United States as a consistent 

 whole, it is necessary to correlate the various local elements. The events of one dis- 

 trict — the succession of eruptions, sedimentary deposits, and erosions — must be con- 

 nected with the synchronous events of other regions. It is especially important to 

 determine the synchrony of deposits. So far as the outcrops of strata can be contin- 

 ously traced, or can be observed at short intervals, correlationcan be effected by the 

 study of stratigraphy alone. The correlation of strata separated by wide intervals 

 of discontinuity can be effected only through the study of their contained fossils. 

 This is not always easy, and it is now generally recognized that it is possible only 

 within restricted limits. As distance increases the refinement in detail of correla- 

 tion diminishes. 



Recent discussions in connection with the work of the International Congress of 

 Geologists have shown that different students assign different limits to the possibili- 

 ties of correlation, and give different weights to the various kinds of paleontologic 

 evidence employed. 



The study of the data and principles of correlation is thus seeu to be a necessary 

 part of the work of the Geological Survey, and by making the study at the present 

 time it can offer a timely contribution to general geologic philosophy. It has there- 

 fore been determined to undertake the preparation of a series of essays summarizing 

 existing knowledge bearing on the correlation of American strata. It is proposed to 

 have a treatise prepared by a competent specialist on each of the following systems : 

 The Quaternary, the Newer Tertiary, the Older Tertiary, the Cretaceous, the Jura- 

 Trias, the Carboniferous, the Devonian, the Silurian, the Cambrian, the Eparchean, 

 and the Archean. 



Each essay will consider the several geographic provinces of the system it treats, 

 the stratigraphic divisions that have been made in the several provinces, the extent 

 to which these divisions can be correlated with one another, the degree of precision 

 with which the upper and lower limits of the system can be correlated with the 

 limits of the corresponding European system, and the extent to which the American 

 subdivisions can be correlated with the European. It is proposed to treat sepa- 



•Nintli Annual lieport of theU. S. Geological Survey, p. 16. 



